At various stages of the integrated circuit fabrication process, it is necessary to polish a surface of the semiconductor wafer. In general, a semiconductor wafer can be polished to remove high typography, surface defects such as crystal lattice damage, scratches, roughness or embedded particles of dirt or dust. This polishing process is often referred to as mechanical planarization and is utilized to improve the quality and reliability of semiconductor devices. The process is usually performed during the formation of various devices and integrated circuits on the wafer. The polishing process may also involve the introduction of a chemical slurry to facilitate higher removal rates and selectivity between films of the semiconductor surface. Such a polishing process is referred to as chemical mechanical planarization (CMP).
In general, the CMP process involves holding and rotating a thin flat wafer of semiconductor material against a wetted polishing surface under controlled pressure and temperature. FIG. 1 depicts a conventional CMP device 10 having a rotatable polishing platen 12, a polishing head assembly 14, and a chemical supply system 16. Platen 12 is rotated at a prescribed velocity by motor 18. Platen 12 is typically covered by a replaceable, relatively soft pad material 20 such as blown polyurethane, which may be wetted with a lubricant such as water.
Polishing head assembly 14 includes a carrier 22 which holds the semiconductor wafer (not shown) adjacent to platen 12. Polishing head assembly 14 further includes motor 24 for rotating carrier/semiconductor wafer 22, and a carrier displacement mechanism 26 which linearly moves carrier/semiconductor wafer 22 radially across platen 12 as indicated by arrows 28 and 30. Carrier assembly 14 applies a controlled downward pressure, P, as illustrated by arrow 32, to carrier/semiconductor wafer 22 to hold the wafer against rotating platen 12.
Chemical supply system 16 introduces a polishing slurry (as indicated by arrow 34) to be used as an abrasive medium between platen 12 and the semiconductor wafer. Chemical supply system 16 includes a chemical storage 36 and a conduit 38 for transferring the slurry from chemical storage 36 to the planarization environment atop platen 12.
One problem encountered in CMP processes is the non-uniform removal of the semiconductor surface. Removal rate is directly proportional to downward pressure on the wafer, rotational speeds of the platen and wafer, slurry particle density and size, slurry composition, and the effective area of contact between the polishing pad and the wafer surface. Further, removal caused by the polishing pad is related to the radial position of the wafer on the platen. The removal rate increases as the semiconductor wafer moves radially (i.e., linearly) outward relative to the platen due to a higher platen rotational velocity. Additionally, removal rates tend to be higher at wafer edge than at wafer center because the wafer edge is rotating at a higher speed than the wafer center.
One approach to addressing the problems associated with non-uniform removal across the platen is presented in U.S. Pat. No. 5,234,867, entitled "Method for Planarizing Semiconductor Wafers with a Non-Circular Polishing Pad." As the title indicates, planarizing apparatus is disclosed therein which includes a non-circular pad, mounted atop the platen, which engages and polishes the surface of the semiconductor wafer. A polishing head displacement mechanism moves the polishing head and semiconductor wafer across and past a peripheral edge of the non-circular pad to effectuate a uniform polish of the semiconductor wafer surface. Wafer movement across the pad is linear or radial and is not controlled by the shape of the pad. The drawback of this planarizing apparatus is that the carrier and wafer are intentionally brought partially outside the edge of the pad, which can be very dangerous to the integrity of the wafer. Further, such action can cause the pad edge to curve upwards or downwards or rip upon repeated use, thereby shortening the life of the pad.
Thus, a need continues to exist in the art for an improved planarization process which significantly reduces the problems associated with non-uniform removal across the platen.